1. Field of the Invention
The present invention relates to a device for detecting the positions of diced semiconductor chips placed on a film in the case where they are die-bonded to a lead frame or the like by means of a die bonding device.
2. Description of the Prior Art
A die bonding device serves to pick up semiconductor chips on a film and to die-bond the same to a lead frame or the like. It is necessary to precisely know the positions of the chips in order to pick up the chips on the film accurately. The die bonding device is provided with an optical position detector for detecting the positions of the chips.
FIG. 4 is a view showing the structure of a conventional chip position detector. A film 51 has chips 52a, 52b . . . placed thereon, and can be moved on a plane with being held by a table (not shown). The movement of the film 51 causes the chips 52a, 52b . . . to be opposed to an industrial television camera (ITV) 55 in order. Light is radiated to the chips 52a, 52b . . . on the film 51 by means of a vertical illuminator 53 and an oblique illuminator 54. The ITV 55 detects the light reflected by the chips 52a, 52b . . . so as to recognize their coordinates.
In the case where the positions of the chips are detected by the above-mentioned conventional method, however, light reflection varies owing to variation in film thickness of an oxide on chip surfaces. Consequently, the positions of the chips cannot always be read precisely. In the case where the semiconductor chip is a LED element (GaP, GaAlAs or the like), particularly, a native oxide varies widely. Consequently, surface conditions are unstable so that the positions of the chips cannot always be detected precisely. This disadvantage will be described in more detail.
FIGS. 5 and 6 are plan and side views of a LED element as a semiconductor chip, respectively. The central portions of the upper faces of LED elements 52a, 52b, 52c . . . are provided with electrodes 61 comprised of Al or the like. An oxide film 62 is formed as a passivation around the electrode 61. The thickness of the oxide film 62 varies. In the case where the positions of the chips are detected by the position detector shown in FIG. 4, they cannot always be detected precisely. By way of example, FIGS. 7(a) to (d) show the state of detection carried out by the ITV 55. Although the LED elements 52a, 52b, 52c . . . are excellent in quality, the state of detection varies. In case of the state of detection shown in FIGS. 7(b) to (d), it is impossible to carry out position detection precisely.